Heavy-wall steel plate having 450mpa-grade tensile strength and excellent resistance to hydrogen induced cracking and method for manufacturing same

ABSTRACT

The present disclosure relates to a heavy-wall steel plate having 450 MPa-grade tensile strength and excellent resistance to hydrogen induced cracking, and a method for manufacturing the same. The heavy-wall steel plate includes, by weight, carbon (C): 0.03% to 0.06%, silicon (Si): 0.2% to 0.4%, manganese (Mn): 1.0% to 1.6%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, copper (Cu): 0.05% to 0.4%, nickel (Ni): 0.05% to 0.5%, calcium (Ca): 0.0005% to 0.003%, a balance of iron (Fe), and other unavoidable impurities, wherein a thickness of the heavy-wall steel plate is 40 mm or more.

CROSS-REFERENCE OF RELATED APPLICATIONS

This application is the U. S. National Phase under 35 U. S. C. § 371 ofInternational Patent Application No. PCT/KR2017/013550, filed on Nov.24, 2017, which in turn claims the benefit of Korean Patent ApplicationNo. 10-2016-0176896, filed Dec. 22, 2016, the entire disclosures ofwhich applications are incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to a heavy-wall steel plate havingexcellent resistance to hydrogen induced cracking, and a method formanufacturing the same, and, in particular, to a normalizing heattreated heavy-wall steel plate having a thickness of 40 mm or more andhaving a tensile strength of 450 MPa, and a method for manufacturing thesame.

BACKGROUND ART

A heavy-wall steel plate for guaranteeing hydrogen induced crackingaccording to American Petroleum Institute (API) standard has been usedfor line pipe, process pipe, or the like, and the required propertiesand manufacturing process of a steel material has been used determined,depending on the usage environment. When the end customer has a hightemperature environment, the manufacturing process of a steel materialrequires a heat treatment process such as a normalizing process, aquenching/tempering process, or the like. Furthermore, whenmanufacturing process of a steel pipe includes the normalizing process,a heat treatment steel plate requires a normalizing steel material.

However, the normalizing steel material is generally low in strength dueto the characteristics of the air-cooling material, and when the contentof the alloying elements such as C, Mn, and the like, increases in orderto facilitate an increase in strength, the resistance to hydrogeninduced cracking may decrease sharply. The reason is that the content ofpearlite in the steel plate increases with the addition of C, Mn, andthe like, and the resistance to hydrogen induced cracking decreasessharply over a certain percentage of the pearlite fraction. In addition,since the resistance to hydrogen induced cracking is reduced after thetubing of the steel pipe due to the characteristics of the normalizedsteel material, the requirements for resistance to hydrogen inducedcracking have become stricter in recent years.

The following technologies have been proposed so far for the productionof normalized steel material for securing the resistance to hydrogeninduced cracking.

Korean Patent Publication No. 2004-0021117 proposes a steel material fora pressure vessel having a tensile strength of 600 MPa, which isexcellent in toughness and used for materials such as boilers of a powerplant, pressure vessels, or the like. The steel material for a pressurevessel proposed by the Patent Publication has a composition comprising,by weight, carbon (C): 0.08% to 0.16%, silicon (Si): 0.1% to 0.4%,manganese (Mn): 0.8% to 1.8%, molybdenum (Mo): 0.2% to 0.8%, nickel(Ni): 0.3% to 0.8%, boron (B): 0.0005% to 0.003%, titanium (Ti): 0.005%to 0.025%, aluminum (Al): 0.01% to 0.08%, phosphorus (P): 0.010% orless, sulfur (S): 0.010% or less, nitrogen (N): 0.010% or less, abalance of iron (Fe), and other unavoidable impurities. The steelmaterial is heat-treated at a temperature in a range of Ac3 to 930° C.,and, then, forcibly cooled to room temperature at a cooling rate of 0.5to 5° C./sec. As described above, the Patent Publication relates to asteel material for a pressure vessel having a tensile strength of 600MPa and a manufacturing method thereof.

However, the components and the manufacturing conditions described inthe above-mentioned Korean Patent Publication No. 2004-0021117 have notbeen able to produce a normalizing steel material excellent inresistance to hydrogen induced cracking due to a high C content.Further, there is a disadvantage that Mo, not effective in improving thestrength of the normalized steel, has been used intentionally therein.In addition, despite the fact that Cu is not used, there is adisadvantage that a relatively large amount of Ni added is added toprevent hot shortness. Moreover, there is a problem that distribution ofinclusions greatly affecting resistance to hydrogen induced cracking ofa low-strength steel material is not considered.

Korean Patent No. 0833070 proposes a heavy-wall steel plate for apressure vessel excellent in resistance to hydrogen induced crackingwhile satisfying a tensile strength of 500 MPa. In the heavy-wall steelplate for a pressure vessel proposed by the above patent, and a methodfor manufacturing the same, a steel material having a compositioncomprising, by weight, carbon (C): 0.1% to 0.30%, silicon (Si): 0.15% to0.40%, manganese (Mn): 0.6% to 1.2%, phosphorus (P): 0.035% or less,sulfur (S): 0.020% or less, aluminum (Al): 0.001% to 0.05%, chromium(Cr): 0.35% or less, nickel (Ni): 0.5% or less, copper (Cu): 0.5% orless, molybdenum (Mo): 0.2% or less, vanadium (V): 0.05% or less,niobium (Nb): 0.05% or less, calcium (Ca): 0.0005% to 0.005%, a balanceof iron (Fe), and other unavoidable impurities, is used. Further, such asteel plate satisfies Equation 1: Cu+Ni+Cr+Mo<1.5%, Equation 2:Cr+Mo<0.4%, Equation 3: V+Nb<0.1%, and Equation 4: Ca/S>1.0, asrelationships for components. The above patent relates to a method formanufacturing the steel material having a tensile strength of 500 MPa,as described above, comprising: reheating the steel material at 1050° C.to 1250° C.; performing a recrystallization controlled rolling operationof hot-rolling the reheated steel material at a temperature not lowerthan a non-recrystallization temperature; and performing a normalizingoperation of heat treating the hot-rolled steel material at atemperature of 850° C. to 950° C. at 1.3×t+(10-30 minutes) (where tdenotes a thickness (mm) of a steel material).

However, since the above-mentioned Korean Patent No. 0833070, as in theKorean Patent Publication No. 2004-0021117, contains Cr, Mo, and V,which are less effective for improving the strength of the normalizedsteel, and, in addition, the C content described therein is 0.1 wt % ormore, there is also a problem in securing the resistance to hydrogeninduced cracking.

DISCLOSURE Technical Problem

The present disclosure is made to solve the above problems of the priorart, and it is an object of the present disclosure to optimizecomponents in steel, a microstructure of the steel, a rolling operation,a cooling operation, and a heat treatment operation, to provide anormalizing heat treated heavy-wall steel plate having excellentresistance to hydrogen induced cracking, having a thickness of 40 mm ormore and having a tensile strength of 450 MPa. In addition, unlike theprior art, the heat treatment operation is performed at a temperaturehigher than that of a conventional normalizing heat treatment operationwithout including expensive precipitation-type elements such as Cr, Mo,V, etc., to provide a normalizing heat treated heavy-wall steel platehaving excellent resistance to hydrogen induced cracking, and having atensile strength of 450 MPa.

The object of the present disclosure is not limited to the abovedescription. Those skilled in the art will appreciate that there will beno difficulty in understanding the present disclosure from the overallcontents of the present disclosure.

Technical Solution

According to an aspect of the present disclosure, a heavy-wall steelplate having excellent resistance to hydrogen induced cracking,includes, by weight, carbon (C): 0.03% to 0.06%, silicon (Si): 0.2% to0.4%, manganese (Mn): 1.0% to 1.6%, phosphorus (P): 0.03% or less,sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N):0.01% or less, copper (Cu): 0.05% to 0.4%, nickel (Ni): 0.05% to 0.5%,calcium (Ca): 0.0005% to 0.003%, a balance of iron (Fe), and otherunavoidable impurities, wherein a thickness of the heavy-wall steelplate is 40 mm or more, and tensile strength of the heavy-wall steelplate is 450 MPa or more.

The heavy-wall steel plate may further include niobium (Nb): 0.005% to0.05% and titanium (Ti): 0.005% to 0.03%.

The heavy-wall steel plate may be a microstructure having a compositestructure of ferrite and pearlite, and an area fraction of the pearlitemay be less than 10%.

The heavy-wall steel plate may further include Al—Ca-based inclusions,and a minimum distance between Al—Ca-based inclusions having a diameterof 2 μm or more may be 100 μm or more in a rolling direction.

According to an aspect of the present disclosure, a method formanufacturing a heavy-wall steel plate having 450 MPa-grade tensilestrength and excellent resistance to hydrogen induced cracking,includes:

preparing a slab having a composition comprising, by weight, carbon (C):0.03% to 0.06%, silicon (Si): 0.2% to 0.4%, manganese (Mn): 1.0% to1.6%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less,aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, copper (Cu):0.05% to 0.4%, nickel (Ni): 0.05% to 0.5%, calcium (Ca): 0.0005% to0.003%, a balance of iron (Fe), and other unavoidable impurities;

heating the slab to 1100° C. to 1300° C.;

hot-rolling the heated slab such that the total rolling reductionthickness is less than 200 mm at a finish rolling temperature of 900° C.or higher, so as to prepare a hot-rolled steel plate; and

subjecting the hot-rolled steel plate to a normalizing heat treatment ata temperature of 1000° C. to 1100° C.

Advantageous Effects

According to an aspect of the present disclosure, by optimizingcomponents in steel, a microstructure of the steel, and a rollingoperation, a steel plate having excellent resistance to hydrogen inducedcracking, having a thickness of 40 mm or more, and having a tensilestrength of 450 MPa, at relatively low manufacturing costs.

DESCRIPTION OF DRAWINGS

FIG. 1 is a graph illustrating distribution of tensile strengthsaccording to normalizing temperatures of Comparative Examples 5 to 10,having the same components as those of Inventive Example 1.

FIG. 2 is a photograph showing Al—Ca-based inclusions in a hydrogeninduced cracking fracture surface of Comparative Example 7(low-temperature rolled material).

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail.

The present inventors have found that Cr, Mo, V, and the like, which arecommonly used in conventional normalized steels, have no effect onstrength improvement, while alloying elements such as C, Si, Mn, Cu, andthe like have a great influence on the strength improvement ofnormalized steel. In addition, considering the fact that C and Mn arethe most influential elements for deterioration of resistance tohydrogen induced cracking, it is possible to secure the strength inheavy-wall steel material with thickness of 40 mm or more by using Siand Cu while limiting C and Mn contents. It has also been found that theadditional effect of Si and Cu may improve soft matrix structure, whichis vulnerable to hydrogen induced cracking of low strength steelmaterials by strengthening the ferrite matrix phase.

The present inventors have found that hydrogen induced crackinggenerated in low strength steel has a close relationship to distributionof inclusions contained in the steel, and consequently, a method ofrestricting the distribution of inclusions has been envisaged.

In general, the normalized steel material has been subjected tohigh-temperature general rolling in terms of manufacturing cost, but hasa characteristic that the finishing rolling temperature decreases as thethickness of the steel material decreases. Also, as a thickness of thesteel material is reduced or a thickness of the slab is increased, thetotal thickness of the steel material for securing the product thicknessmay increase. In the present disclosure, it is proposed that thefinishing rolling temperature is limited to a certain level or more, andthe slab thickness before the slab reheating is optimized, to limit thetotal rolling reduction thickness, on the basis of the findings that, asthe finish rolling temperature decreases or the total rolling reductionthickness increases, resistance to hydrogen induced cracking of thenormalized steel material is reduced.

Also, in the case of normalized steel material, even with the samecomponent, as the thickness of the steel plate increases, the coolingrate may decrease. Therefore, it may not be easy to secure tensilestrength while increasing thickness. In order to secure the strength ofthe normalizing steel material, a method of refining the austenitegrains is generally used in which the content of C, Mn, and the like isadded or the normalizing temperature is decreased. However, when thecontent is added, resistance to hydrogen induced cracking may decrease,and a sufficient increase in strength may not be expected only throughdecreasing the normalizing temperature. In the present disclosure, anormalizing temperature of at least the general level is proposed tomaximize the effect of increasing quenchability due to grain coarsening,based on the fact that austenite grains are coarsened by increasing thenormalizing temperature.

Hereinafter, a component system of a normalized heat-treated steel platehaving a thickness of 40 mm or more and a tensile strength of 450 MPa,which is excellent in resistance to hydrogen induced cracking of thepresent invention, will be described in detail.

C: 0.03 wt % to 0.06 wt %

C is closely related to the manufacturing method together with othercomponents. Among the steel components, C has a greatest influence onthe characteristics of the steel material. When the content of C is lessthan 0.03 wt %, it may be difficult to secure sufficient strength, and awelding heat-affected zone may be softened more than necessary.Meanwhile, when the content of C is more than 0.06 wt %, the resistanceto hydrogen induced cracking of the steel plate may be decreased, andweldability may be deteriorated. Therefore, it is preferable to limitthe content of C to 0.03-0.06 wt %.

Si: 0.2 wt to 0.4 wt %

Si not only acts as a deoxidizer in a steel manufacturing process, butalso serves to raise the strength of the steel material. When thecontent of Si is less than 0.2 wt %, it may be difficult to securesufficient strength. When the content of Si is more than 0.4 wt %,weldability is lowered, and scale peelability is caused upon rolling.Therefore, it is preferable to limit the content to 0.2 wt to 0.4 wt %.

Mn: 1.0 wt to 1.6 wt %

Mn may be an element for improving the strength of the steel by loweringthe ferrite transformation temperature until a certain amount is added,without impairing impact toughness, and is preferably added in an amountof 1.0 wt % or more. When it is added in an amount exceeding 1.6% byweight, there is a problem that center segregation may occur to decreasethe resistance to hydrogen induced cracking sharply. Therefore, thecontent thereof is preferably limited to 1.0 wt % to 1.6 wt %.

P: 0.03 wt % or less

P is an impurity element, and when the content is more than 0.03 wt %,weldability is significantly decreased, and also impact toughness isdecreased, and thus, it is preferable to limit the content to 0.03 wt %or less. In particular, 0.01 wt % or less is more preferable in terms oflow-temperature impact toughness.

S: 0.003 wt % or less

S is also an impurity element, and when the content is more than 0.003wt %, the ductility, impact toughness, and weldability of steel aredecreased. Therefore, it is preferable to limit the content to 0.003 wt% or less. In particular, since S is bonded to Mn to form a MnSinclusion, and decrease the low-temperature impact toughness of steel,0.002 wt % or less is more preferable.

Al: 0.06 wt % or less

Usually, Al may serve as a deoxidizer which reacts with oxygen presentin molten steel to remove oxygen. Therefore, it is general to add Al inan amount to provide a steel material with sufficient deoxidationability. When added more than 0.06 wt %, a large amount of anoxide-based inclusion is formed to inhibit the impact toughness of amaterial, and thus, the content is limited to 0.06 wt % or less.

N: 0.01 wt % or less

Since it is difficult to industrially completely remove N from steel,the upper limit thereof is 0.01 wt % which may be allowed in amanufacturing process. N may form nitrides with Al, Ti, Nb, V, etc., toinhibit austenite crystal grin growth, and to help toughness andstrength improvement, however, when the content thereof exceeds 0.01%, Nis present in a solid-soluble state and N in a solid-soluble state hasan adverse influence on low temperature toughness. Therefore, it ispreferable to limit the content thereof to 0.01% or less.

Cu: 0.05% to 0.4%

Cu may be an element for improving the strength of ferrite through solidsolution strengthening, and should be added in an amount of 0.05% ormore. Since Cu is an element which causes cracks on the surface during ahot-rolling operation to hinder the surface quality, it is preferable torestrict the upper limit thereof to 0.4%.

Ni: 0.05% to 0.5%

Ni may be an element which improves the toughness of steel, and ispreferably added in an amount of 0.05% or more, to reduce surface cracksgenerated during a hot-rolling operation of Cu-added steel. In addition,the Ni content of 0.5% or more may increase price of the steel material.Therefore, it is preferable to restrict the upper limit thereof to 0.5%.

Ca: 0.0005% to 0.003%

Ca may serve to spheroidize MnS inclusions. MnS, an inclusion having arelatively low melting point, produced in the central portion, may bestretched upon rolling to be present as a stretched inclusion in thecentral portion of steel. When MnS is present in a relatively largeamount and partially dense, it may serve to decrease elongation whenstretched in a thickness direction. The added Ca may react with MnS tosurround MnS, thereby interfering with the stretching of MnS. In orderto represent this MnS spheroidizing effect, Ca should be added in anamount 0.0005 wt % or more. Since Ca has high volatility and thus, has arelatively low yield, considering the load produced in the steelmanufacturing process, it is preferable to restrict the upper limitthereof to 0.003 wt % or less.

The steel plate of the present disclosure may further include Nb and Tioptionally in addition to the above-mentioned composition.

Nb: 0.005 to 0.05%

Nb may be solid-solubilized when reheating a slab, and may inhibitaustenite crystal grain growth during a hot rolling operation, and,then, may be precipitated to improve the strength of steel to 0.005% ormore. When Nb is added in an excess amount exceeding 0.05%, it isprecipitated together with Ti in the central portion to induce hydrogeninduced cracking, such that the upper limit of Nb is limited to 0.05% inthe present disclosure.

Ti: 0.005 to 0.03%

Ti may be an element effective in inhibiting the growth of austenitecrystal grains by being bonded to N when reheating the slab to form TiN.When Ti is added in an amount exceeding 0.03%, the low-temperatureimpact toughness of the heat-treated material may deteriorate.Therefore, the upper limit of Ti is limited to 0.03% in the presentdisclosure. From the viewpoint of low-temperature toughness, it is morepreferable to add 0.01% or less.

The steel plate of the present disclosure may further include Fe andunavoidable impurities, and does not exclude the addition of othercomponents in addition to the above-described components. For example,the steel plate of the present disclosure may additionally include othercomponents in addition to the above-mentioned components in thecomposition of steel.

The steel having the above composition may have differentmicrostructures depending on the contents of the elements, rollingoperations, cooling conditions, and heat treatment conditions, and mayaffect strength and resistance to hydrogen induced cracking depending onthe microstructure even with the same composition. Hereinafter, amicrostructure of a normalized steel material of the present disclosure,having excellent resistance to hydrogen induced cracking, having athickness of 40 mm or more, and having a tensile strength of 450 MPa,will be described.

Matrix Structure: Complex Structure of Ferrite and Pearlite

The steel plate having excellent resistance to hydrogen induced crackingaccording to the present disclosure may be a steel plate having athickness of 40 mm or more, and may be a steel plate having excellent inresistance to hydrogen induced cracking while maintaining a relativelyhigh strength of 450 MPa or more in tensile strength, regardless of itsthickness. In general, a normalized steel has two phases of ferrite andpearlite as its matrix structure without adding excessive components.When a pearlite fraction in the matrix structure is 10% or more, sinceresistance to hydrogen induced cracking is lowered, the pearlitefraction in the present disclosure may be limited to less than 10%.

Minimum Distance Between Al—Ca-Based Inclusions Having Diameter of 2 μmor More: 100 μm or More

The Al—Ca-based inclusions may be a factor deteriorating the resistanceto hydrogen induced cracking of low strength steel. When the minimumdistance between Al—Ca-based inclusions having a diameter of 2 μm ormore in a rolling direction is less than 100 μm, the resistance tohydrogen induced cracking may be deteriorated. It is preferable that alower limit in the minimum distance between the Al—Ca-based inclusionshaving a diameter of 2 μm or more be limited to 100 μm.

Next, a method of manufacturing a normalized heat-treated steel plate ofthe present disclosure, having excellent resistance to hydrogen inducedcracking, having a thickness of 40 mm or more, and having a tensilestrength of 450 MPa, will be described.

First, in the present disclosure, a steel slab having theabove-mentioned composition may be prepared, and, then, may be reheatedin a temperature range of 1100° C. to 1300° C.

The reheating process is an operation of heating the steel slab to arelatively high temperature, to hot-roll the steel slab. When thereheating temperature is higher than the upper limit of 1300° C. definedby the present disclosure, the austenite crystal grains may beexcessively coarsened to lower the strength of steel, and to generatescale defects. When the reheating temperature is less than 1100° C.,re-solid soluble ratio of the alloying elements may decrease.Accordingly, in the present disclosure, the range of the reheatingtemperature is preferably limited to 1100° C. to 1300° C., and morepreferably 1100° C. to 1180° C. in terms of strength and toughness.

In the present disclosure, the heated slab may be hot-rolled such thatthe total rolling reduction thickness is less than 200 mm at a finishrolling temperature of 900° C. or higher, so as to prepare a hot-rolledsteel plate.

The lower the finish rolling temperature is, the finer the crystalgrains are. Therefore, the low-temperature toughness of the steel may beimproved. However, when the finish rolling temperature is lower than900° C., large Al—Ca-based inclusions may be divided in the rollingdirection, such that a minimum distance between Al—Ca-based inclusionshaving a diameter of 2 μm or more is less than 100 μm. Therefore, sincethe resistance to hydrogen induced cracking in the steel may be rapidlydeteriorated, it is preferable to hot-roll the heated slab that thetotal rolling reduction thickness in the present disclosure is limitedto be less than 200 mm.

In the case of a Thermo-Mechanical Controlling Process (TMCP) material,as the total rolling reduction thickness of the slab increases, thecrystal grains may be finer and the low-temperature toughness may beimproved. When the total rolling reduction thickness of the slab is 200mm or more, the Al—Ca-based inclusions of a normalizing steel materialmay be easily divided in the rolling direction during a rollingoperation, such that a minimum distance between Al—Ca-based inclusionshaving a diameter of 2 μm or more is less than 100 μm. Therefore, sincethe resistance to hydrogen induced cracking in the steel may be rapidlydeteriorated, it is preferable to hot-roll the heated slab that thetotal rolling reduction thickness in the present disclosure is limitedto be 200 mm or less.

In the present disclosure, the hot-rolled steel plate may be cooled,preferably by air cooling. Since the steel material to be provided issubjected to a heat treatment after rolling, the cooling process is notan important process variable, but when the steel plate is water cooledfrom a relatively high temperature, it may cause shape deformation andproductivity resistance of the steel plate.

In the present disclosure, the hot-rolled steel plate is subjected to anormalizing treatment in a temperature range of 1000° C. to 1100° C.

The normalizing temperature refers to a temperature at which the cooledsteel plate is reheated to the austenite region at a certain temperatureor more after the hot-rolling operation, and an air cooling operationmay perform after the heating operation. In general, the normalizingtemperature may be performed directly on the Ar3 temperature. Since thenormalizing temperature range proposed in this study is aimed atcoarsening crystal grain through the austenite crystal grain growth, itmay deviate from the normal normalizing temperature.

In the present disclosure, when the normalizing temperature is less than1000° C., the austenite crystal grains may be not sufficientlycoarsened. Therefore, no sufficient quenchability may be secured at thetime of the air cooling operation, and ferrite and pearlite formed atthe time of the air cooling operation may not be completely transformedinto austenite phase. When the normalizing temperature exceeds 1100° C.,the austenite crystal grains may be excessively coarsened. Therefore,the low-temperature toughness may deteriorate and a high-temperaturescale may be caused on the surface of the steel. In consideration ofthis, in the present disclosure, the range of the normalizing reheatingtemperature is preferably limited to 1000° C. to 1100° C.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described more specificallyby way of examples. It should be noted, however, that the followingexamples are intended to illustrate and specify the present disclosure,and not to limit the scope of the present disclosure. This is becausethe scope of the present disclosure is determined by the mattersdescribed in the claims and the matters reasonably deduced therefrom.

Example

Steel slabs having the composition illustrated in the following Table 1were reheated, hot-rolled, and normalized to produce steel plates. Inthe following Tables 2 and 3, inventive examples comply with the steelcomposition and the manufacturing conditions according to an aspect ofthe present disclosure, and comparative examples deviate from any one ofthe steel composition and the manufacturing conditions according to anaspect of the present disclosure.

The steel types illustrated in the following Table 1 were prepared toproduce steel plates according to the manufacturing process conditionsillustrated in the following Table 2. Specifically, the steel slabhaving the composition illustrated in the following Table 1 was heatedto the heating temperature illustrated in the following Table 2, rolledto the finish rolling temperature and the total rolling reductionthickness illustrated in the following Table 2, reheated to thereheating temperature illustrated in the following Table 2, and thenair-cooled.

A pearlite area fraction, a distance between the Al—Ca-based inclusions,tensile strength, and a hydrogen induced cracking sensitivity, e.g., acrack length ratio (CLR) were measured for the thus prepared steelplate, and the results are illustrated in the following Table 3.

The pearlite area fraction and the distance between the Al—Ca-basedinclusions were obtained by observing the microstructure of the steelplate, and the hydrogen induced cracking sensitivity (CLR) was testedaccording to the method specified by a National Association of CorrosionEngineers (NACE), and percentage of the length of the hydrogen inducedcracking generated with respect to the entire length of the specimen.

The values listed in the following Table 1 refer to weight percent.Comparative Examples 1 to 4 are comparative examples in which thecomponents having steel composition and the manufacturing processconditions fail to satisfy the ranges according to an aspect of thepresent disclosure, and Comparative Examples 5 to 10 are comparativeexamples in which the components having steel composition satisfy theranges according to an aspect of the present disclosure, but themanufacturing process conditions fail to satisfy the ranges according toan aspect of the present disclosure.

TABLE 1 Steel C Si Mn P S Al N Cr Mo Cu Ni Mb Ti V Ca 1 0.041 0.31 1.320.007 0.0008 0.03 0.005 0.31 0.24 0.02 0.01 0.0015 2 0.038 0.32 1.340.008 0.0007 0.029 0.004 0.29 0.22 0.01 0.0013 3 0.068 0.25 1.51 0.0080.0008 0.041 0.005 0.19 0.14 0.2 0.23 0.006 0.008 0.02 0.001 4 0.0430.22 1.2 0.008 0.0008 0.041 0.005 0.27 0.12 0.014 0.013 0.012 0.0013 50.048 0.25 1.75 0.008 0.0009 0.033 0.005 0.18 0.09 0.08 0.013 0.010.0014 6 0.043 0.12 1.35 0.008 0.0008 0.029 0.007 0.18 0.25 0.012 0.030.0011 *The remainder in Table 1 is Fe and unavoidable impurities.

TABLE 2 Total Rolling Heat Reduction Temp. Finish Rolling ThicknessNormalizing Example (° C.) Temp. (° C.) (mm) Temp. (° C.) Inventive 11168 977 188 1035 Example 2 1159 966 176 1023 Comparative 1 1165 990 192915 Example 2 1152 975 188 942 3 1145 935 179 928 4 1144 964 167 925 51133 891 193 931 6 1121 876 196 931 7 1137 835 184 931 8 1122 955 179980 9 1160 952 185 900 10 1160 973 240 1020

TABLE 3 Hydrogen Pearlite Al—Ca-based Induced Area Inclusion TensileCracking Fraction Minimum Distance Strength Sensitivity Steel Example(%) (μm) (MPa) (CLR, %) 1 *IE1 5.2 332 468 0 2 IE2 5.1 430 471 0.1 3**CE1 12.5 266 457 4.8 4 CE2 3.6 343 387 0 5 CE3 5.8 136 466 12.6 6 CE46.1 144 384 0 1 CE5 5.2 86 435 3.5 CE6 5.3 63 444 10.7 CE7 5.1 35 45632.5 CE8 5 361 385 0 CE9 5.3 345 428 0 CE10 5.8 92 461 1.2 *IE:Inventive Example, **CE: Comparative Example

Referring to Tables 1 to 3 above, Inventive Examples 1 and 2 satisfyingthe steel composition and the manufacturing process conditions accordingto an aspect of the present disclosure, have a tensile strength of 450MPa or more and a hydrogen induced cracking sensitivity (CLR) of 1% orless, and, thus, it can be seen that resistance to hydrogen inducedcracking thereon is excellent.

Comparative Examples 1 to 10, which fail to satisfy one of the componentsystem, component range, and process conditions according to an aspectof the present disclosure, have a tensile strength of less than 450 MPa,or a hydrogen induced cracking sensitivity (CLR) exceeding 1%, and,thus, it can be seen that resistance to hydrogen induced crackingthereon was not sufficient.

As reported above, it can be seen that a steel plate having excellentresistance to hydrogen induced cracking, having a thickness of 40 mm ormore, and having a tensile strength of 450 MPa, may be obtained bymanufacturing the steel plate according to the composition andmanufacturing process of the present disclosure.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentdisclosure as defined by the appended claims.

1. A heavy-wall steel plate having excellent resistance to hydrogen induced cracking, comprising, by weight, carbon (C): 0.03% to 0.06%, silicon (Si): 0.2% to 0.4%, manganese (Mn): 1.0% to 1.6%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, copper (Cu): 0.05% to 0.4%, nickel (Ni): 0.05% to 0.5%, calcium (Ca): 0.0005% to 0.003%, a balance of iron (Fe), and other unavoidable impurities, wherein a thickness of the heavy-wall steel plate is 40 mm or more, and tensile strength of the heavy-wall steel plate is 450 MPa or more.
 2. The heavy-wall steel plate according to claim 1, further comprising niobium (Nb): 0.005% to 0.05% and titanium (Ti): 0.005% to 0.03%.
 3. The heavy-wall steel plate according to claim 1, wherein the heavy-wall steel plate is a microstructure having a composite structure of ferrite and pearlite, and an area fraction of the pearlite is less than 10%.
 4. The heavy-wall steel plate according to claim 1, wherein the heavy-wall steel plate further comprises Al—Ca-based inclusions, and a minimum distance between Al—Ca-based inclusions having a diameter of 2 μm or more is 100 μm or more in a rolling direction.
 5. The heavy-wall steel plate according to claim 1, wherein a hydrogen induced cracking sensitivity of the heavy-wall steel plate has a crack length ratio (CLR) of 1% or less.
 6. A method for manufacturing a heavy-wall steel plate having 450 MPa-grade tensile strength and excellent resistance to hydrogen induced cracking, comprising: preparing a slab having a composition comprising, by weight, carbon (C): 0.03% to 0.06%, silicon (Si): 0.2% to 0.4%, manganese (Mn): 1.0% to 1.6%, phosphorus (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, copper (Cu): 0.05% to 0.4%, nickel (Ni): 0.05% to 0.5%, calcium (Ca): 0.0005% to 0.003%, a balance of iron (Fe), and other unavoidable impurities; heating the slab to 1100° C. to 1300° C.; hot-rolling the heated slab such that the total rolling reduction thickness is less than 200 mm at a finish rolling temperature of 900° C. or higher, so as to prepare a hot-rolled steel plate; and subjecting the hot-rolled steel plate to a normalizing heat treatment at a temperature of 1000° C. to 1100° C.
 7. The method according to claim 6, wherein the heavy-wall steel plate is a microstructure having a composite structure of ferrite and pearlite, and an area fraction of the pearlite is less than 10%.
 8. The method according to claim 6, wherein the heavy-wall steel plate further comprises Al—Ca-based inclusions, and a minimum distance between Al—Ca-based inclusions having a diameter of 2 μm or more is 100 μm or more in a rolling direction. 